In a conventional horizontal type continuous casting machine, a mold is horizontally connected to an opening in a lower portion of a side wall of a tundish for the horizontal type continuous casting machine. FIG. 1 is a schematic vertical sectional view illustrating an example of the junction between a tundish for receiving molten steel and a conventional mold in a conventional horizontal type continuous casting machine. As shown in FIG. 1, the mold 1 is horizontally connected, through a front nozzle 2, a feed nozzle 3 and a break ring 4, to the opening in the lower portion of the side wall 5 of the tundish. One end of the front nozzle 2 is inserted into the opening in the lower portion of the side wall 5 of the tundish, and the other end of the front nozzle 2 is in contact with one end of the feed nozzle 3. The other end of the feed nozzle 3 is in contact with one end of the break ring 4, and the other end of the break ring 4 is in contact with an inner bore 6 at an inlet end of the mold 1. Thus, the opening in the side wall 5 of the tundish, the front nozzle 2, the feed nozzle 3, the break ring 4 and the inner bore 6 of the mold 1 form a horizontal passage for molten metal. The mold 1 is covered by a jacket 7, and a space 8 is formed between the mold 1 and the jacket 7. Cooling water is circulated through the space 8 to cool the mold 1.
Molten steel received in the tundish is withdrawn into a cast steel strand through the mold 1. For the purpose of preventing a very thin solidified shell of the cast steel strand formed near the break ring 4 from breaking, and the solidified shell from sticking to the inner surface of the mold 1, the cast steel strand is intermittently and continuously withdrawn from the mold 1 in the horizontal direction by means of a plurality of cycles each comprising one pull and one push.
FIG. 2 is a descriptive view illustrating an example of the above-mentioned cycle comprising one pull and one push for withdrawing the cast steel strand in the horizontal direction from the mold. In FIG. 2, the abscissa represents time, and the ordinate indicates a pulling speed of the cast steel strand in the portion above point 0 and a pushing speed of the cast steel strand in the portion below point 0. In FIG. 2, the portion "a" represents a pull period in one cycle comprising one pull and one push, the portion "b" represents the last stage of the pull period in the above-mentioned cycle, and the portion "c" represents a push period in the above-mentioned cycle. The distance of one pull in one cycle for withdrawing the cast steel strand is longer than the distance of one push. In the push period "c", the cast steel strand is slightly pushed back in the direction opposite to the withdrawal direction of the cast steel strand in order to prevent cracks from occurring on the surface portion of the solidified shell of the cast steel strand along with shrinkage of the solidified shell of the cast steel strand.
FIGS. 3(A) to 3(C) are partial sectional views illustrating the formation of a solidified shell 10 of the cast steel strand 9 in the inner bore 6 of a conventional mold 1 when intermittently and continuously withdrawing the cast steel strand 9 in the horizontal direction from the mold 1 by means of the above-mentioned method. FIG. 3(A) illustrates the formation of the solidified shell 10 of the cast steel strand 9 during a pull period in one cycle comprising one pull and one push, FIG. 3(B) illustrates the formation of the solidified shell 10 of the cast steel strand 9 during the last stage of the pull period in this cycle, and FIG. 3(C) illustrates the formation of the solidified shell 10 of the cast steel strand 9 during a push period in this cycle. Intermittent withdrawal of the cast steel strand 9 has the effect of causing a thin solidified shell 10 formed near the break ring 4 during the pull period in one cycle to grow thicker during the push period in this cycle as shown in FIG. 3(C), so as to prevent the solidified shell 10 from breaking during the next pull period in the next one cycle.
However, since the cast steel strand 9 is intermittently and continuously withdrawn from the mold 1 by means of a plurality of cycles each comprising one pull and one push, a junction face is produced in the solidified shell 10 of the cast steel strand 9 between a unit shell 10' formed during one cycle comprising one pull and one push and another unit shell 10" formed during the next one cycle comprising also one pull and one push, as shown in FIGS. 3(A) to 3(C). This junction face is known as a cold shut 11. The above-mentioned cold shut 11 poses no problem so far as it is completely welded, but if it is incompletely welded, a crack may occur, in the mold 1, on the surface portion of the solidified shell 10 of the cast steel strand 9 along the cold shut 11 during the pull period in one cycle for withdrawing the cast steel strand 9, and remains as a flaw on the surface of the cast steel strand 9. This flaw usually has a depth of from 0.5 to 1.5 mm.
Now, the reason for the formation of an incompletely welded cold shut 11 will be described below.
As shown in FIG. 1 and FIGS. 3(A) to 3(C), the inner bore 6 of the conventional mold 1 has a uniform transverse sectional area over the entire length of the mold 1 from the inlet end to the exit end thereof, and the wall of the mold 1 has a uniform thickness. As previously mentioned, the mold 1 is cooled by cooling water circulating through the space 8 formed between the mold 1 and the jacket 7, and the break ring 4 which is in contact with the inner bore 6 of the mold 1, is also cooled by the thus cooled mold 1. Therefore, the corner portion 10a of the unit shell 10' (herein referred to as the "corner portion of the unit shell"), which is in contact with the corner formed by the mold 1 and the break ring 4 (hereinafter referred to as the "corner of the inner bore 6"),is cooled more remarkably than the other portion of the unit shell 10', which is in contact only with the mold 1, by means of both the mold 1 and the break ring 4 during the push period in one cycle for withdrawing the cast steel strand 9, and, as a result, the temperature of the corner portion 10a of the unit shell 10' is largely reduced.
FIG. 4 is a graph illustrating the decrease in temperature of the corner portion 10a of the unit shell 10', which is in contact with the corner of the inner bore 6 of the conventional mold 1. As shown in FIG. 4, the temperature of the corner portion 10a of the unit shell 10' is largely reduced during a very short period of time of only from 0.1 to 0.3 second for which the corner portion 10a of the unit shell 10' stays in the corner of the inner bore 6. When the temperature of the corner portion 10a of the unit shell 10', which is formed during one cycle for withdrawing the cast steel strand 9, is low, the unit shell 10", which is newly formed during the next one cycle, is not completely welded together with the corner portion 10a of the preceding unit shell 10' . According to experience, when the temperature of the corner portion 10a of the unit shell 10' becomes up to 1,400.degree. C., the corner portion 10a of the preceding unit shell 10' can no longer be completely welded together with the newly formed unit shell 10". As a result, an incompletely welded cold shut 11 is produced between the unit shell 10' having a low-temperature corner portion 10a, which is formed during one cycle comprising one pull and one push for withdrawing the cast steel strand 9, on the one hand, and the unit shell 10", which is formed during the next one cycle, on the other hand.
In general, when the number of cycles each comprising one pull and one push for withdrawing the cast steel strand 9 from the mold 1 is larger than 150 cycles/minute, the cold shuts 11 are completely welded, and no cracks occur on the surface portion of the cast steel strand 9 along the cold shuts 11. However, increasing the number of cycles to over 150 cycles/minute causes a heavier load acting on the withdrawal facilities of the cast steel strand 9 including pinch rolls. The number of cycles is thus practically limited to the range of from 50 to 150 cycles/minute. When the number of cycles is within the range of from 50 to 150 cycles/minute, incompletely welded cold shuts 11 are produced for the reason as mentioned above, and cracks occur on the surface portion of the cast steel strand 9 along the incompletely welded cold shuts 11.
Incompletely welded cold shuts are also produced when casting a molten metal other than molten steel into a cast metal strand by the horizontal type continuous casting machine.
Under such circumstances there is a strong demand for development of a mold for horizontally and continuously casting molten metal into a cast metal strand, which, when casting molten metal into the cast metal strand by a horizontal type continuous casting machine, permits prevention of cracks from occurring along cold shuts produced on the surface portion of a solidified shell of the cast metal strand, but a mold for horizontally and continuously casting molten metal into a cast metal strand provided with such characteristics has not as yet been proposed.